How is an isolation transformer sized?
Choosing the rating of the small transformer at every light: big enough for the fixture, close enough to its load to run efficiently, and matched to the series circuit.
Every light on a series circuit sits behind its own isolation transformer, which couples the fixture to the loop. Transformer sizing picks the rating of that transformer, and because there is one at every light, a small habit repeated hundreds of times shapes the whole circuit load.
1Cover the fixture load
The rating must be at or above the power the fixture draws on the secondary, plus anything else connected there. The figure to use is the fixture’s real draw, not just its lamp label, and for LED fixtures it is far smaller than for the halogen lamp they replace. Standard ratings run in steps (FAA AC 150/5345-47 lists 10/15, 20/25, 30/45, 65, 100, 150, 200, 300 and 500 W), so take the nearest size at or above the real draw rather than an exact match.
| Sizing | What happens in the transformer | Effect on the circuit |
|---|---|---|
| Too small | Overloaded: runs hot, output sags, lamp dims, insulation and life suffer | Light output falls and the transformer may fail early |
| Well matched | Near its rating: best efficiency and power factor for the power delivered | Lowest loss, so the lightest load on the constant current regulator (CCR) |
| Much too large | Lightly loaded: low efficiency, magnetising current large for the power delivered | Extra loss on every unit adds to the circuit load |
2Stay close to the load
Bigger is not safer. A transformer carrying far less than its rating still draws its magnetising current, so its efficiency and power factor fall. That cost is paid at every light, and it comes back as extra load on the regulator. An overloaded transformer is worse: it overheats and the lamp is dimmer than it should be.
3Match primary and secondary
The primary is wired into the series loop, so it must suit the circuit current, commonly 6.6 A, and the high insulation class of the loop. The secondary voltage and current must suit the lamp or LED driver. All of this is fixed by the nameplate, so sizing is also about reading it correctly. Allow for a likely future lamp change, such as halogen to LED, and note the transformer’s result for the load calculation.
- Find the fixture load. Use the fixture data for the lamp or LED module and any extra secondary load.
- Choose a rating. Take the nearest standard rating at or above that load.
- Match the primary. Series circuit current and insulation class.
- Match the secondary. Voltage and current for the lamp or LED driver.
- Check the loading. Close to rating, not heavily oversized, not overloaded.
- Record the result. Pass the transformer input power into the load calculation.
| Term | Meaning |
|---|---|
| Rating | Power the transformer is built to deliver, in W or VA |
| Loading | Fixture load as a share of the transformer rating |
| Primary | Winding connected to the series loop |
| Secondary | Winding that feeds the fixture |
| Magnetising current | Current that magnetises the core, drawn even at light load |
| Efficiency | Output power as a share of input power, best near rating |
Frequently Asked Questions
Q1What is transformer sizing in airfield lighting?
Choosing the power rating of the isolation transformer that feeds each fixture from the series circuit, so it comfortably supplies the fixture load without being so large that it runs inefficiently.
Q2What does the transformer rating have to cover?
At least the power the fixture draws on its secondary, plus any other load on that secondary. The rating is in watts or volt-amperes and sits on the transformer nameplate.
Q3Why not fit a much bigger transformer to be safe?
A lightly loaded transformer is less efficient and draws more magnetising current for the power it delivers. Across a whole circuit those extra losses add to the load the regulator must carry.
Q4What happens if the transformer is too small?
It runs hot, its output is pulled down so the lamp is dimmer than it should be, and its insulation and life suffer. Overloading is the more serious error of the two.
Q5Do LED fixtures need a different transformer?
LED fixtures draw much less than halogen lamps, so the load is small. Use the transformer the fixture manufacturer specifies, and check how it behaves when lightly loaded, rather than assuming the old rating still fits.
Q6What else must match besides power?
The primary must suit the series circuit current, usually 6.6 A, and the high insulation class of the loop, and the secondary voltage and current must suit the lamp or LED driver. These are set on the nameplate.
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